1. Field of the Invention
The present invention relates to the field of well perforating.
2. Description of the Prior Art
During the completion of an oil or gas well, a length of casing is cemented in a borehole, and then one or more zones of the casing are perforated to communicate the bore of the casing with subsurface geological formations intersected by the borehole so that oil or gas from that subsurface formation may be produced by the well.
One well-known type of perforating system is a tubing conveyed perforating system wherein the perforating guns and related apparatus are carried by a tubing string made up of a plurality of threaded joints of tubing or pipe which are connected together and lowered into the well. These tubing conveyed completion systems may be run in combination with a drill stem test string so that the well can be perforated and tested in a single trip.
In some situations, it is desirable to be able to selectively perforate more than one zone of the well at different times. The prior art has typically addressed this need by the provision of multiple firing heads which are constructed to actuate at different operating pressures. With these systems, the selection of the appropriate firing head and gun to be fired is determined by the pressure which is applied to the tubing string or the well annulus to actuate the firing head. Systems of this type capable of firing several perforating guns independently during one trip into the well can be constructed utilizing the Time Delayed Firing Head available from Vann Systems of Carrollton, Tex. The Vann Time Delayed Firing Head utilizes a set of shear pins the number of which can be selected to determine the actuating pressure of each firing head.